Slit2-Robo4 receptor responses inhibit ANDV directed permeability of human lung microvascular endothelial cells

Slit2-Robo4 signal pathway and tight junction in intestine mediate LPS-induced inflammation in mice

BACKGROUND

Sepsis is one of the most common clinical diseases, which is characterized by a serious and uncontrollable inflammatory response. LPS-induced inflammation is a critical pathological event in sepsis, but the underlying mechanism has not yet been fully elucidated.

METHODS

The animal model was established for two batches. In the first batch of experiments, Adult C57BL/6J mice were randomly divided into control group and LPS (5 mg/kg, i.p.)group . In the second batch of experiments, mice were randomly divided into control group, LPS group, and LPS+VX765(10 mg/kg, i.p., an inhibitor of NLRP3 inflammasome) group. After 24 hours, mice were anesthetized with isoflurane, blood and intestinal tissue were collected for tissue immunohistochemistry, Western blot analysis and ELISA assays.

RESULTS

The C57BL/6J mice injected with LPS for twenty-four hours could exhibit severe inflammatory reaction including an increased IL-1β, IL-18 in serum and activation of NLRP3 inflammasome in intestine. The injection of VX765 could reverse these effects induced by LPS. These results indicated that the increased level of IL-1β and IL-18 in serum induced by LPS is related to the increased intestinal permeability and activation of NLRP3 inflammasome. In the second batch of experiments, results of western blot and immunohistochemistry showed that Slit2 and Robo4 were significant decreased in intestine of LPS group, while the expression of VEGF was significant increased. Meanwhile, the protein level of tight junction protein ZO-1, occludin, and claudin-5 were significantly lower than in control group, which could also be reversed by VX765 injection.

CONCLUSIONS

In this study, we revealed that Slit2-Robo4 signaling pathway and tight junction in intestine may be involved in LPS-induced inflammation in mice, which may account for the molecular mechanism of sepsis.

Binding of the Andes Virus Nucleocapsid Protein to RhoGDI Induces the Release and Activation of the Permeability Factor RhoA

Andes virus (ANDV) nonlytically infects pulmonary microvascular endothelial cells (PMECs), causing acute pulmonary edema termed hantavirus pulmonary syndrome (HPS). In HPS patients, virtually every PMEC is infected; however, the mechanism by which ANDV induces vascular permeability and edema remains to be resolved. The ANDV nucleocapsid (N) protein activates the GTPase RhoA in primary human PMECs, causing VE-cadherin internalization from adherens junctions and PMEC permeability. We found that ANDV N protein failed to bind RhoA but coprecipitates RhoGDI (Rho GDP dissociation inhibitor), the primary RhoA repressor that normally sequesters RhoA in an inactive state. ANDV N protein selectively binds the RhoGDI C terminus (residues 69 to 204) but fails to form ternary complexes with RhoA or inhibit RhoA binding to the RhoGDI N terminus (residues 1 to 69). However, we found that ANDV N protein uniquely inhibits RhoA binding to an S34D phosphomimetic RhoGDI mutant. Hypoxia and vascular endothelial growth factor (VEGF) increase RhoA-induced PMEC permeability by directing protein kinase Cα (PKCα) phosphorylation of S34 on RhoGDI. Collectively, ANDV N protein alone activates RhoA by sequestering and reducing RhoGDI available to suppress RhoA. In response to hypoxia and VEGF-activated PKCα, ANDV N protein additionally directs the release of RhoA from S34-phosphorylated RhoGDI, synergistically activating RhoA and PMEC permeability. These findings reveal a fundamental edemagenic mechanism that permits ANDV to amplify PMEC permeability in hypoxic HPS patients. Our results rationalize therapeutically targeting PKCα and opposing protein kinase A (PKA) pathways that control RhoGDI phosphorylation as a means of resolving ANDV-induced capillary permeability, edema, and HPS. IMPORTANCE HPS-causing hantaviruses infect pulmonary endothelial cells (ECs), causing vascular leakage, pulmonary edema, and a 35% fatal acute respiratory distress syndrome (ARDS). Hantaviruses do not lyse or disrupt the endothelium but dysregulate normal EC barrier functions and increase hypoxia-directed permeability. Our findings reveal a novel underlying mechanism of EC permeability resulting from ANDV N protein binding to RhoGDI, a regulatory protein that normally maintains edemagenic RhoA in an inactive state and inhibits EC permeability. ANDV N sequesters RhoGDI and enhances the release of RhoA from S34-phosphorylated RhoGDI. These findings indicate that ANDV N induces the release of RhoA from PKC-phosphorylated RhoGDI, synergistically enhancing hypoxia-directed RhoA activation and PMEC permeability. Our data suggest inhibiting PKC and activating PKA phosphorylation of RhoGDI as mechanisms of inhibiting ANDV-directed EC permeability and therapeutically restricting edema in HPS patients. These findings may be broadly applicable to other causes of ARDS.

ER stress-related molecules induced by Hantaan virus infection in differentiated THP-1 cells

Endoplasmic reticulum stress (ER stress) can be induced by virus infection. In this part, we explored whether Hantaan virus (HTNV) infection could induce ER stress in differentiated THP-1 (dTHP-1) cells. It showed that the mRNA and protein levels of ER stress-related 78 kDa glucose-regulated protein (GRP78, HSPA5) and mRNA levels of X box-binding protein 1 (XBP-1), activating transcription factor 6(ATF6) and PKR-like ER kinase (PERK) after HTNV infection, were significantly higher than that in uninfected control group. However, the mRNA levels of C/EBP homologous protein (CHOP), glucose-regulated protein 94 (GRP94, HSPC4), and inositol-requiring enzyme1 (IRE1) were not significantly different between the infected group and the untreated group in 2 h after virus infection. It is unusual in activating GRP78 but not GRP94. Meanwhile, dTHP-1 cells infected with HTNV at 12 h did not show obvious apoptosis. These results indicated that the HTNV infection could induce the unfolded protein response (UPR) in dTHP-1 cells, without directly leading to cell apoptosis during 12 h after virus infection.

The potential of Slit2 as a therapeutic target for central nervous system disorders

Introduction: Slit2 is an extracellular matrix protein that regulates migration of developing axons during central nervous system (CNS) development. Roundabout (Robo) receptors expressed by various cell types in the CNS, mediate intracellular signal transduction pathways for Slit2. Recent studies indicate that Slit2 plays important protective roles in a myriad of processes such as cell migration, immune response, vascular permeability, and angiogenesis in CNS pathologies. Areas covered: This review provides an overview of the diverse functions of Slit2 in CNS disorders and discusses the potential of Slit2 as a therapeutic target. We reviewed preclinical studies reporting the role of Slit2 in various CNS disease models, transgenic animal research, and rodent models that utilized Slit2 as a therapy. Expert opinion: Slit2 exerts a wide array of beneficial effects ranging from anti-migration, blood-brain barrier (BBB) protection, inhibition of peripheral immune cell infiltration, and anti-apoptosis in various disease models. However, a dual role of Slit2 in endothelial permeability has been observed in transgenic animals. Further research on Slit2 will be crucial including key issues such as effects of transgenic overexpression versus exogenous Slit2, function of Slit2 dependent on cellular expression of Robo receptors and the underlying pathology for potential clinical translation.

Regulatory mechanisms of Robo4 and their effects on angiogenesis

Roundabout4 (Robo4) is a transmembrane receptor that belongs to the Roundabout (Robo) family of axon guidance molecules. Robo4 is an endothelial-specific receptor that participates in endothelial cell migration, proliferation, and angiogenesis and the maintenance of vasculature homeostasis. The purpose of this review is to summarize and analyze three main mechanisms related to the expression and function of Robo4 during developmental and pathological angiogenesis. In this review, static shear stress and the binding of transcription factors such as E26 transformation-specific variant 2 (ETV2) and Slit3 induce Robo4 expression and activate Robo4 during tissue and organ development. Robo4 interacts with Slit2 or UNC5B to maintain vascular integrity, while a disturbed flow and the expression of transcription factors in inflammatory or neoplastic environments alter Robo4 expression levels, although these changes have uncertain functions. Based on the mechanisms described above, we discuss the aberrant expression of Robo4 in angiogenesis-related diseases and propose antiangiogenic therapies targeting the Robo4 signaling pathway for the treatment of ocular neovascularization lesions and tumors. Finally, although many problems related to Robo4 signaling pathways remain to be resolved, Robo4 is a promising and potentially valuable therapeutic target for treating pathological angiogenesis and developmental defects in angiogenesis.

Inhibitory Effect of Slit2-N on VEGF165-induced proliferation of vascular endothelia via Slit2-N-Robo4-Akt pathway in choroidal neovascularization

Researches have been focusing on the role of Slit2 in angiogenesis, specifically in cell migration and vessel permeability. Nevertheless, the role of Slit2-N, the bioactive fragment of Slit2, in the proliferation of vascular endothelia in choroidal neovascularization and some related mechanisms have not been studied yet. Thus, our study aimed to explore the role of Slit2-N in proliferation of vascular endothelia and the related mechanisms in choroidal neovascularization. Fluorescein isothiocyanate perfusion and HE staining were performed to evaluate volumes of choroidal neovascularization lesions. The effect of Slit2-N on VEGF165-induced cell proliferation and some related mechanisms were detected by CCK8 assay, flow cytometry, siRNA transfection, and western blotting. We found that Slit2-N reduced volumes of laser-induced choroidal neovascularization networks in vivo. Results of the in vitro study showed Slit2-N reduced VEGF165-induced cell proliferation of both human umbilical vascular endothelial cells and human microvascular endothelial cells possibly via activation of AKT rather than that of ERK1/2. Additionally, Robo4, one of the receptors binding to Slit2-N, was involved in the inhibitory effect of Slit2-N. Generally, our findings revealed the inhibitory role of Slit2-N in proliferation of vascular endothelia and some related mechanisms, and presented some potential targets, molecules along Slit2-N-Robo4-AKT axis, to choroidal neovascularization therapy.

Antagonistic effects of exogenous Slit2 on VEGF-induced choroidal endothelial cell migration and tube formation

Vascular endothelial growth factor (VEGF) is involved in the pathogenesis of choroidal neovascularization. The aim of the present study was to assess the effects of exogenous slit guidance ligand 2 (Slit2) on VEGF-induced choroidal endothelial cell (CEC) migration and tube formation. The protein and mRNA expression levels of Slit2, roundabout guidance receptor (Robo) 1 and Robo4 in CECs were evaluated by immunocytochemistry and reverse transcription-polymerase chain reaction analyses, respectively. Western blot analysis was used to assess Robo4 protein levels in CECs exposed to increasing concentrations (0, 50, 75, 100, 125 and 150 ng/ml) of exogenous Slit2. The effects of exogenous Slit2 (125 ng/ml) on VEGF-induced CEC migration and tube formation were also examined. CECs expressed Slit2 and Robo4, but lacked Robo1 expression, at the mRNA and protein levels. Robo4 protein expression increased significantly following treatment with 50–150 ng/ml exogenous Slit2. No significant difference in Robo4 protein expression was observed in CECs treated with 125 and 150 ng/ml Slit2. VEGF-induced CEC migration and tube formation were significantly reduced following treatment with 125 ng/ml exogenous Slit2. In conclusion, these results indicate that Robo4 is expressed in CECs. In addition, exogenous Slit2 may regulate Robo4 expression and partially inhibit VEGF-induced CEC migration and tube formation.

Slit2/Robo4 signaling pathway modulates endothelial hyper-permeability in a two-event in vitro model of transfusion-related acute lung injury

Transfusion-related acute lung injury (TRALI) remains the leading cause of transfusion-related mortality. Endothelium semipermeable barrier function plays a critical role in the pathophysiology of transfusion-related acute lung injury (TRALI). Recently, Roundabout protein 4 (Robo4), interaction with its ligand Slit 2, was appreciated as a modulator of endothelial permeability and integrity. However, not much is known about the role of Slit2/Robo4 signaling pathway in the pathophysiology of TRALI. In this study, the TRALI model was performed by the "two-event" model of polymorphonuclear neutrophils (PMN)-mediated pulmonary microvascular endothelial cells (PMVECs) damage. We investigated the expression of Slit2/Robo4 and VE-cadherin and examined the pulmonary endothelial hyper-permeability in TRALI model. We found that the expression of Slit2/Robo4 and VE-cadherin were significantly decreased in a time-dependent manner, whereas the PMVECs permeability was gradually increased over time in TRALI model. Moreover, the treatment with Slit2-N, an active fragment of Slit2, increased the expression of Slit2/Robo4 and VE-cadherin to protect PMVECs from PMN-mediated pulmonary endothelial hyper-permeability. These results indicate that targeting Slit2/Robo4 signaling pathway may modulate the permeability as well as protect the integrity of endothelial barrier. In addition, Slit2-N appears to be a promising candidate for developing novel therapies against TRALI.

Recombinant Slit2 Reduces Surgical Brain Injury Induced Blood Brain Barrier Disruption via Robo4 Dependent Rac1 Activation in a Rodent Model

Brain tissue surrounding surgical resection site can be injured inadvertently due to procedures such as incision, retractor stretch, and electrocauterization when performing neurosurgical procedures, which is termed as surgical brain injury (SBI). Blood brain barrier (BBB) disruption due to SBI can exacerbate brain edema in the post-operative period. Previous studies showed that Slit2 exhibited vascular anti-permeability effects outside the brain. However, BBB protective effects of Slit2 following SBI has not been evaluated. The objective of this study was to evaluate whether recombinant Slit2 via its receptor roundabout4 (Robo4) and the adaptor protein, Paxillin were involved in reducing BBB permeability in SBI rat model. Our results showed that endogenous Slit2 increased in the surrounding peri-resection brain tissue post-SBI, Robo4 remained unchanged and Paxillin showed a decreasing trend. Recombinant Slit2 administered 1 h before injury increased BBB junction proteins, reduced BBB permeability, and decreased neurodeficits 24 h post-SBI. Furthermore, recombinant Slit2 administration increased Rac1 activity which was reversed by Robo4 and Paxillin siRNA. Our findings suggest that recombinant Slit2 reduced SBI-induced BBB permeability, possibly by stabilizing BBB tight junction via Robo4 mediated Rac1 activation. Slit2 may be beneficial for BBB protection during elective neurosurgeries.

Cocaine enhances HIV-1 gp120-induced lymphatic endothelial dysfunction in the lung

Pulmonary complications are common in both AIDS patients and cocaine users. We addressed the cellular and molecular mechanisms by which HIV and cocaine may partner to induce their deleterious effects. Using primary lung lymphatic endothelial cells (L-LECs), we examined how cocaine and HIV-1 gp120, alone and together, modulate signaling and functional properties of L-LECs. We found that brief cocaine exposure activated paxillin and induced cytoskeletal rearrangement, while sustained exposure increased fibronectin (FN) expression, decreased Robo4 expression, and enhanced the permeability of L-LEC monolayers. Moreover, incubating L-LECs with both cocaine and HIV-1 gp120 exacerbated hyperpermeability, significantly enhanced apoptosis, and further impaired in vitro wound healing as compared with cocaine alone. Our studies also suggested that the sigma-1 receptor (Sigma-1R) and the dopamine-4 receptor (D4R) are involved in cocaine-induced pathology in L-LECs. Seeking clinical correlation, we found that FN levels in sera and lung tissue of HIV(+) donors were significantly elevated as compared to HIV(-) donors. Our in vitro data demonstrate that cocaine and HIV-1 gp120 induce dysfunction and damage of lung lymphatics, and suggest that cocaine use may exacerbate pulmonary edema and fibrosis associated with HIV infection. Continued exploration of the interplay between cocaine and HIV should assist the design of therapeutics to ameliorate HIV-induced pulmonary disorders within the drug using population.

Roundabout 4 regulates blood-tumor barrier permeability through the modulation of ZO-1, Occludin, and Claudin-5 expression

The blood-tumor barrier (BTB) restricts the delivery of chemotherapeutic drug molecules to tumor tissues. We found that the endothelial cell (EC) receptor molecule Roundabout 4 (Robo4) is endogenously expressed in human brain microvascular ECs and that it is upregulated in a BTB model of glioma cocultured ECs. Knockdown of Robo4 in this BTB model increased permeability; short hairpin RNA targeting Robo4 (shRobo4) led to decreased transendothelial electric resistance values, increased BTB permeability, and downregulated expression of the EC tight junction proteins ZO-1, occludin, and claudin-5. Roundabout 4 influenced BTB permeability via binding with its ligand, Slit2. Short hairpin RNA targeting Robo4 also increased matrix metalloproteinase-9 (MMP-9) activity and expression in glioma cocultured ECs; pretreatment with the MMP inhibitor GM6001 partially blocked the effects of shRobo4 on the transendothelial electric resistance values and ZO-1 and occludin expression. Short hairpin RNA targeting Robo4 also upregulated the phosphorylation of Src and Erk1/2; the Src inhibitor PP2 and the Erk1/2 inhibitor PD98059 blocked shRobo4-mediated alteration in ZO-1 and occludin expression. Together, our results indicate that knockdown of Robo4 increased BTB permeability by reducing EC tight junction protein expression, and that the Src-Erk1/2-MMP-9 signal pathways are involved in this process. Thus, Robo4 may represent a useful future therapeutic target for enhancing BTB permeability.

Raised plasma Robo4 and cardiac surgery-associated acute kidney injury

OBJECTIVE

Endothelial dysfunction associated with systemic inflammation can contribute to organ injury/failure following cardiac surgery requiring cardiopulmonary bypass (CPB). Roundabout protein 4 (Robo4), an endothelial-expressed transmembrane receptor and regulator of cell activation, is an important inhibitor of endothelial hyper-permeability. We investigated the hypothesis that plasma levels of Robo4 are indicative of organ injury, in particular acute kidney injury (AKI), after cardiac surgery.

METHODS

Patients (n = 32) undergoing elective cardiac surgery with CPB were enrolled, prospectively. Plasma Robo4 concentrations were measured pre-, 2 and 24 h post-operatively, using a commercially available ELISA. Plasma and endothelial markers of inflammation [interleukin (IL) -6, -8, -10: von Willibrand factor (vWF) and angiopoeitin-2 (Ang-2)] and the AKI marker, neutrophil gelatinase-associated lipocalin (NGAL), were also measured by ELISA.

RESULTS

Plasma Robo4 increased significantly (p<0.001) from pre-operative levels of 2515 ± 904 pg/ml to 4473 ± 1915 pg/ml, 2 h after surgery; and returned to basal levels (2682 ± 979 pg/ml) by 24 h. Plasma cytokines, vWF and NGAL also increased 2 h post-operatively and remained elevated at 24 h. Ang-2 increased 24 h post-operatively, only. There was a positive, significant correlation (r = 0.385, p = 0.0298) between Robo-4 and IL-10, but not other cytokines, 2 h post-operatively. Whilst raised Robo4 did not correlate with indices of lung dysfunction or other biomarkers of endothelial activation; there was a positive, significant correlation between raised (2 h) plasma NGAL and Robo4 (r = 0.4322, p = 0.0135). When patients were classed as AKI or non-AKI either using NGAL cut-off of 150 ng/ml, or the AKI Network (AKIN) clinical classification; plasma Robo4 was significantly higher (p = 0.0073 and 0.003, respectively) in AKI vs. non-AKI patients (NGAL cut-off: 5350 ± 2191 ng/ml, n = 16 vs. 3595 ± 1068 pg/ml, n = 16; AKIN: 6546 pg/ml, IQR 5025-8079, n = 6; vs. 3727 pg/ml, IQR 1962-3727, n = 26) subjects.

CONCLUSION

Plasma Robo4 levels are increased, transiently, following cardiac surgery requiring CPB; and higher levels in patients with AKI suggest a link between endothelial dysregulation and onset of AKI.

Virus interactions with endothelial cell receptors: implications for viral pathogenesis

The endothelial lining of the vasculature performs a vital role in maintaining fluid barrier functions despite balancing nutrient and fluid content of tissues, repairing localized damage, coordinating responses of a plethora of factors, immune cells and platelets through a multitude of endothelial cell surface receptors. Viruses that nonlytically cause lethal hemorrhagic or edematous diseases engage receptors on vascular and lymphatic endothelial cells, altering normal cellular responses that control capillary leakage and fluid clearance functions with lethal consequences. Recent studies indicate that receptors directing dengue virus and hantavirus infection of the endothelium contribute to the dysregulation of normal endothelial cell signaling responses that control capillary permeability and immune responses that contribute to pathogenesis. Here we present recent studies of virally altered endothelial functions that provide new insight into targeting barrier functions of the endothelium as a potential therapeutic approach.

Vascular events in viral hemorrhagic fevers: a comparative study of dengue and hantaviruses

Viral hemorrhagic diseases are a group of systemic viral infections with worldwide distribution and are significant causes of global mortality and morbidity. The hallmarks of viral hemorrhagic fevers are plasma leakage, thrombocytopenia, coagulopathy and hemorrhagic manifestations. The molecular mechanisms leading to plasma leakage in viral hemorrhagic fevers are not well understood. A common theme has emerged in which a complex interplay between pathogens, host immune response, and endothelial cells leads to the activation of endothelial cells and perturbation of barrier integrity. In this article, two clinically distinct viral hemorrhagic fevers caused by dengue viruses and hantaviruses are discussed to highlight their similarities and differences that may provide insights into the pathogenesis and therapeutic approach.

Hantavirus interferon regulation and virulence determinants

Hantaviruses predominantly replicate in primary human endothelial cells and cause 2 diseases characterized by altered barrier functions of vascular endothelium. Most hantaviruses restrict the early induction of interferon-β (IFNβ) and interferon stimulated genes (ISGs) within human endothelial cells to permit their successful replication. PHV fails to regulate IFN induction within human endothelial cells which self-limits PHV replication and its potential as a human pathogen. These findings, and the altered regulation of endothelial cell barrier functions by pathogenic hantaviruses, suggest that virulence is determined by the ability of hantaviruses to alter key signaling pathways within human endothelial cells. Our findings indicate that the Gn protein from ANDV, but not PHV, inhibits TBK1 directed ISRE, kB and IFNβ induction through virulence determinants in the Gn cytoplasmic tail (GnT) that inhibit TBK1 directed IRF3 phosphorylation. Further studies indicate that in response to hypoxia induced VEGF, ANDV infection enhances the permeability and adherens junction internalization of microvascular and lymphatic endothelial cells. These hypoxia/VEGF directed responses are rapamycin sensitive and directed by mTOR signaling pathways. These results demonstrate the presence of at least two hantavirus virulence determinants that act on endothelial cell signaling pathways: one that regulates antiviral IFN signaling responses, and a second that enhances normal hypoxia-VEGF-mTOR signaling pathways to facilitate endothelial cell permeability. These findings suggest signaling pathways as potential targets for therapeutic regulation of vascular deficits that contribute to hantavirus diseases and viral protein targets for attenuating pathogenic hantaviruses.

Hypoxia induces permeability and giant cell responses of Andes virus-infected pulmonary endothelial cells by activating the mTOR-S6K signaling pathway

Andes virus (ANDV) is a South American hantavirus that causes a highly lethal hantavirus pulmonary syndrome (HPS) characterized by hypoxia, thrombocytopenia, and vascular leakage leading to acute pulmonary edema. ANDV infects human pulmonary microvascular and lymphatic endothelial cells (MECs and LECs, respectively) and nonlytically enhances the permeability of interendothelial cell adherence junctions in response to vascular endothelial growth factor (VEGF). Recent findings also indicate that ANDV causes the formation of giant endothelial cells. Here, we demonstrate that hypoxic conditions alone enhance permeability and giant cell responses of ANDV-infected MECs and LECs through activation of the mTOR signaling pathway. In contrast to infection of cells with nonpathogenic Tula virus (TULV), we observed that exposure of ANDV-infected MECs and LECs to hypoxic conditions resulted in a 3- to 6-fold increase in monolayer permeability and the formation of giant cells 3× to 5× normal size. ANDV infection in combination with hypoxic conditions resulted in the enhancement of hypoxia-inducible factor 1α (HIF1α)-directed VEGF A, angiopoietin 4, and EGLN3 transcriptional responses. Constitutive mTOR signaling induces the formation of giant cells via phosphorylation of S6K, and mTOR regulates hypoxia and VEGF A-induced cellular responses. We found that S6K was hyperphosphorylated in ANDV-infected, hypoxia-treated MECs and LECs and that rapamycin treatment for 1 h inhibited mTOR signaling responses and blocked permeability and giant cell formation in ANDV-infected monolayers. These findings indicate that ANDV infection and hypoxic conditions enhance mTOR signaling responses, resulting in enhanced endothelial cell permeability and suggest a role for rapamycin in therapeutically stabilizing the endothelium of microvascular and lymphatic vessels during ANDV infection.